James W. Rector, Bruce P. Marion, Bernard Widrow
A new borehole seismic technique, the Tomex Survey, uses the seismic emanations produced by a drill bit during drilling as a downhole energy source. No downhole instrumentation is required for collection of the seismic data, and the data recording does not interfere with the drilling process. The drill-bit-generated signals are recorded with sensors attached to the top of the drillstring and at various surface-geophone locations near the well. The sensor output at the top of the drillstring is used as a pilot signal for cross-correlation with the signals recorded at the surface-geophone positions. Cross-correlation is used to determine arrival times and to enhance the signal-to-noise ratio of drill-bit-generated events. Deconvolution and time shifts are performed to remov the effects of recording the pilot signal at a location distant from the location of the energy source at the drill bit. A direct comparison between data collected using a drill-bit source and conventional VSP data shows that the drill-bit source yielded comparable data quality.
In using the drill bit as a downhole seismic source for inverse VSP, many of the limitations in conventional VSP are overcome. Several applications for VSP that were previously considered by some explorationists to be prohibitively expensive are now feasible. Furthermore, this measure-while-drill technique offers the potential for the explorationists to make real-time drilling decisions on site.
AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.